Add outline to object
You would have to render a Freestyle pass with the Line Set Silhouette selected on the left and Crease selected on the right, and change to Logical AND on the right.

This would trace a border stroke around the visible perimeter of the object, like tracing the alpha edge.

You'd have to composite this over your OpenGL render, as you can't generate Freestyle render passes when doing OpenGL renders.



In order to make it easier to composite over your OpenGL renders, it would be helpful to make the background and cube the same colour (let's pick pure white), and then make the stroke a black colour. To do this uncheck Transparent in the Film panel of the Render settings section. Make the world a solid white colour and the cube an emission shader with 100% white. Then make the stroke colour in Freestyle a 100% black.